Premium
Dosimetric properties of a scanned beam microtron at low monitor unit settings: Importance for conformal therapy
Author(s) -
Humm John L.,
Larsson Anders,
Lief Eugene P.
Publication year - 1996
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.597662
Subject(s) - microtron , monitor unit , physics , isocenter , flatness (cosmology) , scattering , optics , dosimetry , beam (structure) , photon , linear particle accelerator , atomic physics , foil method , nuclear physics , nuclear medicine , materials science , imaging phantom , medicine , cosmology , quantum mechanics , composite material
The dosimetric stability, linearity, dose rate dependence, and flatness of both photon and electron beams have been evaluated for a racetrack microtron at low monitor unit settings. For photons, the variation in dosimetric output about the mean is <0.4% at 100 monitor units (MU), <1% at 10 MU, and <4% at 2 MU. The output dependence on the dose rate varied by <0.6% between 85–300 MU/min. Flatness and symmetry for the 25‐ and 50‐MV beams showed deviations of <3% at both d max and 10‐cm depths, and only slightly ≳3% at 20 cm, even at only 3 MU, in contrast with other scanned beam accelerators. Broad electron beams on the microtron are created by the superposition of the elementary beam pulses either directly from the scan magnets, or after their broadening through a scattering foil. The dosimetric instability both with and without the foil was less than 0.6% for both the 25‐ and 50‐MeV electrons. Dose nonlinearity was <1% above 10 MU. Field flatness was determined for scan matrices designed to produce a flat field both with and without a scattering foil. Symmetry and flatness deviations were <3% for both electron energies when a scattering foil was used, even for a single scan. The variation of the electron dose per monitor unit between dose rates of 85–300 MU/min was <1% (25 MeV) and <4% (50 MeV) when a scattering foil was used, but as high as 22% (25 MeV) and 36% (50 MeV) for broad beams generated by elementary beam pulses directly from the scanning magnets. The microtron exhibits dosimetric properties which fulfill the recommendations of Task Groups 21 and 25. Based on the stability of the scanned beam at low monitor unit settings, the microtron can be used for 3‐D conformal therapy with both photons and electrons.